Table Of ContentSejong Open Cluster Survey (SOS) - IV. The Young Open
Clusters NGC 1624 and NGC 1931
Beomdu Lim1,5, Hwankyung Sung2, Michael S. Bessell3, Jinyoung S. Kim4, Hyeonoh Hur2,
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1 and Byeong-Gon Park1
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n [email protected]
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5 1Korea Astronomy and Space Science Institute, 776 Daedeokdae-ro, Yuseong-gu, Daejeon
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v 305-348, Korea
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2Department of Astronomy and Space Science, Sejong University, 209 Neungdong-ro,
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Gwangjin-gu, Seoul 143-747, Korea
3Research School of Astronomy and Astrophysics, Australian National University, MSO,
Cotter Road, Weston, ACT 2611, Australia
4StewardObservatory, University ofArizona, 933N.CherryAve. Tucson, AZ85721-0065,
USA
5Corresponding author, Korea Research Council of Fundamental Science & Technology
Research Fellow
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ABSTRACT
Young open clusters located in the outer Galaxy provide us with an oppor-
tunity to study star formation activity in a different environment from the solar
neighborhood. We present a UBVI and Hα photometric study of the young
open clusters NGC 1624 and NGC 1931 that are situated toward the Galactic
anticenter. Various photometric diagrams are used to select the members of the
clusters and to determine the fundamental parameters. NGC 1624 and NGC
1931 are, on average, reddened by hE(B − V)i = 0.92 ± 0.05 and 0.74 ± 0.17
mag, respectively. The properties of the reddening toward NGC 1931 indicate
an abnormal reddening law (R = 5.2 ± 0.3). Using the zero-age main se-
V,cl
quence fitting method we confirm that NGC 1624 is 6.0 ± 0.6 kpc away from
the Sun, whereas NGC 1931 is at a distance of 2.3±0.2 kpc. The results from
isochrone fitting in the Hertzsprung-Russell diagram indicate the ages of NGC
1624 and NGC 1931 to be less than 4 Myr and 1.5 – 2.0 Myr, respectively. We
derived the initial mass function (IMF) of the clusters. The slope of the IMF
(Γ = −2.0±0.2 and Γ = −2.0±0.1) appears to be steeper than
NGC 1624 NGC 1931
that of the Salpeter/Kroupa IMF. We discuss the implication of the derived IMF
based on simple Monte-Carlo simulations and conclude that the property of star
formation in the clusters seems not to be far different from that in the solar
neighborhood.
Subject headings: open clusters and associations: individual (NGC 1624, NGC 1931)
– local interstellar matter – stars: luminosity function, mass function
– 3 –
1. INTRODUCTION
Young open clusters are useful objects to study the star formation process because
about 80 percent of the stars in star forming regions (SFRs) are found in clusters with
at least 100 members (Lada & Lada 2003; Porras et al. 2003). The relation between star
formation activity and environmental conditions is one of the most interesting issues
in star formation studies (Caramazza et al. 2008, 2012). A basic diagnostic tool for
understanding star formation processes is the stellar initial mass function (IMF). The
concept was first introduced by Salpeter (1955). If the stellar IMF has a universal shape
in all SFRs, there must be a fundamental factor controlling the star formation process.
A number of efforts have been devoted to confirming the universality (see review of
Bastian et al. 2010), however, there is as yet no firm conclusion concerning the universality
or diversity of the shape. In this context, the IMF of young open clusters formed in various
star-forming environments may give a clue to the dependence of star formation processes
on environmental conditions. As part of an attempt to study this issue, we investigated the
young open clusters NGC 1624 and NGC 1931 in the outer Galaxy, which is probably a
low-metallicity environment, according to the gradient of metal abundance in the Galactic
disk (Yong et al. 2012).
The young open cluster NGC 1624 is surrounded by the H II region Sh2-212 with
a shell like structure. The cluster comprises several high-mass main sequence (MS)
stars, as well as a large number of pre-main sequence (PMS) stars (Jose et al. 2011).
The most luminous star is NGC 1624-2 (O7f?p – Walborn et al. 2010; Sota et al. 2011)
located near the cluster center. This star with strong X-ray emission is known to be a
slowly rotating star with a very strong magnetic field (Wade et al. 2012). Out of all the
probable PMS members one-fifth have circumstellar disks (Jose et al. 2011). From their
radio observations, Deharveng et al. (2008) found an ultra-compact H II (UCHII) region
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at the border of Sh2-212. A high-mass PMS star (No. 228 of Deharveng et al. 2008)
seems to be associated with the UCHII. A few photometric studies of NGC 1624 provided
the fundamental parameters (Moffat et al. 1979; Chini & Wink 1984; Sujatha & Babu
2006; Deharveng et al. 2008; Jose et al. 2011). The reddening of the cluster is about
E(B − V) = 0.7 – 0.9 mag. The previously determined distance to the cluster was in
the range 6.0 – 6.5 kpc (Moffat et al. 1979; Sujatha & Babu 2006; Deharveng et al. 2008;
Jose et al. 2011), while Chini & Wink (1984) obtained a far different value of 10.3 kpc. The
IMF of the cluster was investigated by Sujatha & Babu (2006) and Jose et al. (2011), and
its slope was reasonably consistent with the Salpeter/Kroupa IMF (Salpeter 1955; Kroupa
2001, 2002). It implies that although NGC 1624 is located in the outer Galaxy, the nature
of the star formation activity is similar to that found in the solar neighbourhood.
The nearer and younger open cluster NGC 1931 is associated with the glowing
nebula Sh2-237. A few bright MS stars and many PMS stars constitute the cluster
(Pandey et al. 2013a). The main ionizing source of the bright nebula is thought to be
the B0.5 star (Glushkov et al. 1975) or two B2 stars (Pandey et al. 2013a). A number of
PMS star candidates were identified by Pandey et al. (2013a) from the Two Micron All
Sky Survey (2MASS; Skrutskie et al. 2006) catalog and Spitzer InfraRed Array Camera
(IRAC) photometry. The spatial distribution of members seems to be divided into two
stellar groups, a northern and a southern group (Bonatto & Bica 2009; Pandey et al.
2013a). The fundamental parameters of the cluster were obtained from photometric
studies (Moffat et al. 1979; Pandey & Mahra 1986; Bhatt et al. 1994; Bonatto & Bica
2009; Pandey et al. 2013a). According to these studies, the reddening and distance of
the cluster are E(B − V) = 0.5 – 1.0 mag and d = 1.8 – 2.4 kpc, respectively. A recent
study (Pandey et al. 2013a) investigated the properties of dust toward the cluster using
polarimetric and photometric data. The reddening law toward the cluster estimated from
the Serkowski law (Serkowski et al. 1975) and color excess ratios indicated that the size
– 5 –
distribution of dust grains may be different from that of the general diffuse interstellar
medium (ISM). They also derived the IMF of the northern, southern, and the entire
cluster regions, respectively. The slope of the IMF appears to be shallower than the
Salpeter/Kroupa IMF.
The present work on NGC 1624 and NGC 1931 is the fifth paper of the Sejong Open
cluster Survey (SOS) project which was initiated to provide homogeneous photometric data
for many open clusters. The overview of the SOS project can be found in Sung et al. (2013a)
(hereafter Paper 0). Comprehensive studies of several open clusters NGC 2353, IC 1848,
and NGC 1893 were published as part of the project (Lim et al. 2011, 2014a,b). In this
work, we revisit the reddening law as well as the fundamental parameters of these clusters
in a homogeneous manner. The IMF of the clusters is also studied in order to investigate
the property of star formation activity in the outer Galaxy. The observation and reliability
of our photometry are described in Section 2. In Section 3, we present several fundamental
parameters of the clusters obtained from photometric diagrams. The reddening law toward
two clusters is also discussed in this section. We construct the Hertzsprung-Russell diagram
(HRD) in Section 4 and derive the IMF of the clusters in Section 5. Several discussions on
the age spread of PMS stars are made in Section 6. Finally, the comprehensive results from
this study are summarized in Section 7.
2. OBSERVATION
2.1. AZT-22 1.5m Telescope of Maidanak Astronomical Observatory
The observations of NGC 1624 was made on 2006 November 24, using the AZT-22
1.5m telescope (f/7.74) at Maidanak Astronomical Observatory (MAO) in Uzbekistan. All
imaging data were acquired using Fairchild 486 CCD camera (SNUCam; Im et al. 2010)
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with the standard Bessell UBVI filters (Bessell 1990). The field of view (FOV) is about
18.′1×18.′1. The characteristics of the CCD chip have been described by Lim et al. (2008)
in detail. The observations comprised 8 frames that were taken in two sets of exposure
times for each band – 3 and 60s in I, 3 and 180s in V, 5 and 300s in B, and 15 and 600s
in U. To transform instrumental magnitudes to standard magnitudes and colors we made
observations of several equatorial standard stars (Menzies et al. 1991) over a wide air mass
range of X ∼ 1.2 – 2.0 on the same night. The observations are summarized in Table 1,
and the left-hand panel in Figure 1 displays the finder chart (dashed line) for the observed
stars brighter than V = 18 mag.
All pre-processing to remove instrumental artifacts was carried out using the
IRAF1/CCDRED packages as described in Lim et al. (2008). Simple aperture photometry
was performed for the standard stars with an aperture size of 14.0 arcsec (26.3 pixels).
Atmospheric extinction coefficients were determined from the photometric data of the
standard stars with a weighted least-square method. The coefficients are presented with
photometric zero points in Table 2. Using IRAF/DAOPHOT we conducted point spread
function (PSF) photometry on stars in the target images using a small fitting radius of 1
full width at half-maximum (FWHM; ≤ 1.0 arcsec), and then aperture correction was done
using the aperture photometry of bright, isolated stars with a photometric error smaller
than 0.01 mag in individual target images. The instrumental magnitude was transformed
to the standard magnitude and colors using the transformation equations below (Paper 0):
M = m −(k −k C)·X +η ·C +α ·UˆT +ζ (1)
λ λ 1λ 2λ λ λ λ
1Image Reduction and Analysis Facility is developed and distributed by the National
Optical Astronomy Observatories, which is operated by the Association of Universities for
Research in Astronomy under operative agreement with the National Science Foundation.
– 7 –
where M , m , k , k , η , C, X, α , UˆT, and ζ are the standard magnitude, instrumental
λ λ 1λ 2λ λ λ λ
magnitude, the primary extinction coefficient, the secondary extinction coefficient,
transformation coefficient, relevant color, air mass, time-variation coefficient, time difference
relative to midnight, and photometric zero point, respectively. We adopted the SNUCam
transformation coefficients (η ), which were recently refined from those in Lim et al.
λ
(2009).
2.2. Kuiper 61” Telescope of Steward Observatory
The observations of NGC 1624 and NGC 1931 were made on 2011 October 29, using
the Kuiper 61” telescope (f/13.5) of Steward Observatory (SO) on Mt. Bigelow in Arizona,
USA. We obtained images with the Mont4k CCD camera and 5 filters (Bessell U, Harris
BV, Arizona I, and Hα). The FOV is about 9.′7×9.′7. Later, additional images of NGC
1931 were taken on 2013 February 5, without the Hα filter. Since typical seeing was 1′.′0 –
2′.′0 (7 – 14 pixels), the 3×3 binning mode (0′.′42 per pixel) allowed us not only to obtain
an appropriate FWHM (∼ 3 pixels) for PSF photometry but also to decrease the readout
time (∼ 10s). In order to obtain reliable transformation relations for this photometric
system across a wide color range, we observed several extreme red and blue standard stars
(Kilkenny et al. 1998) as well as a large number of equatorial standard stars (Menzies et al.
1991) at air masses of 1.2 – 2 during the run. In addition, a few Landolt standard star
fields (Rubin 149, 152, and PG 2213 – Landolt 1992), which contain both extreme red and
blue stars within a small region, were also observed to obtain the secondary extinction
coefficients. The observations are summarized in Table 1, and the finder chart (solid line)
for the stars brighter than V = 18 mag is shown in Figure 1.
The Mont4k photometric system of the Kuiper 61” telescope was used in this survey
project for the first time. Understanding the characteristics of the photometric system
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is essential for obtaining reliable photometric data. We address the characteristics and
transformation relations for the Mont4k CCD photometric system in the Appendix.
Atmospheric extinction coefficients and photometric zero points obtained from the
aperture photometry of the standard stars are presented in Table 2. PSF photometry and
aperture correction for the images of NGC 1624 and NGC 1931 were carried out using
the same procedures as described in the previous section. The instrumental magnitude
was transformed to the standard magnitude and colors using the transformation equations
addressed in the Appendix.
2.3. The Consistency and Completeness of the Photometric Data
We have confirmed from a series of studies (Lim et al. 2011, 2014a,b) that the
photometric data obtained from the photometric system of the AZT-22 1.5m telescope
at MAO are well-tied to the Johnson-Cousins standard system. But we need to check
the homogeneity of the photometric data obtained with the Mont4k CCD camera of the
Kuiper 61” telescope. Since NGC 1624 was observed with the two different photometric
systems, it was possible for us to compare our photometric data directly. Figure 2 shows
the comparison between a few sets of modern CCD photometry. Bold dots (black) in the
upper panels represent the differences between the photometric data obtained at MAO and
SO. We confirmed that our photometry is in good agreement within 0.01 mag (see also
Table 3). The imaging data for NGC 1931 were taken with the Mont4k CCD camera of the
Kuiper 61” telescope at two different epochs. The internal consistency of the two data sets
was examined as shown by bold dots in the lower panels of Figure 2. The photometry is
well consistent with each other within 0.01 mag. We averaged the data sets for each cluster
using a weighted average scheme (Sung & Lee 1995).
A couple of photometric studies with a modern CCD camera have been made for
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NGC 1624 (e.g. Sujatha & Babu 2006; Jose et al. 2011). The combined photometric data
for the cluster were compared with those of the previous studies as marked by squares
and triangles in the upper panels of Figure 2 (see also Table 3). The photometric data of
Sujatha & Babu (2006) are far different from ours, particularly the V magnitudes. All their
colors are systematically redder than the others. There is also a color-dependent trend for
U −B ≤ 0.7.
The difference between the photometry of Jose et al. (2011) and ours is acceptable
in V and B − V, whereas the other colors show systematic differences. Although their
U −B color is in a good agreement with ours for U −B ≤ 0.4, for redder stars their U −B
colors are systematically redder than ours. Their V −I color appears bluer than that of
the others. We note that the previous sets of CCD data for NGC 1624 are likely to involve
problems in the photometric standardization. On the other hand, the photometry of NGC
1931 by Pandey et al. (2013a) is in good agreement with ours. We present a summary of
the comparison in Table 3.
Statistical analysis based on photometric data requires the completeness of the
photometry to be at least 90 percent. In the case of sparse open clusters, the completeness
limit is not very different from that obtained in nearby field region, and that relates to
the turnover magnitude in the luminosity function of the observed stars. We estimated
the completeness of our photometry from the turnover in the luminosity function of all
the observed stars, by assuming a linear slope across the entire magnitude range. Our
photometry is complete down to V = 19 mag (∼ 2 M ) for NGC 1624 and 20 mag
⊙
(∼ 1 M ) for NGC 1931. However, the completeness limit may be an upper limit because
⊙
the bright nebulae surrounding the young open clusters may prevent the detection of faint
stars. The photometric data from this work are available in the electronic tables (Table 4
and 5) or from the authors (BL or HS).
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3. PHOTOMETRIC DIAGRAMS
Photometric studies for young open clusters are based on the two-color diagram
(TCD) and color-magnitude diagrams (CMDs). With well-calibrated empirical relations
and stellar evolution models, fundamental parameters, such as reddening, distance, and
age, can be determined from these photometric diagrams. The fundamental parameters
allow us to investigate the local spiral arm structure in the Galaxy (Paper 0) as well as
the properties and evolution of the dust grain through the reddening law (Pandey et al.
2003; Lim et al. 2011, 2014a,b; Sung & Bessell 2014, Paper 0, etc). In addition, the age
distribution of stars can be used to study the star formation history within a star-forming
region (Palla & Stahler 1999; Sicilia-Aguilar et al. 2004; Sung & Bessell 2010; Lim et al.
2014b, etc) and the pattern speed of the Galactic spiral arms (Dias & L´epine 2005). These
studies should be based on reliable membership selection. In the following sections, we
present membership selection criteria, the reddening law, and fundamental parameters of
NGC 1624 and NGC 1931 with TCDs and CMDs.
3.1. The Extent of NGC 1624 and NGC 1931
As most open clusters are unbound stellar systems, i.e. expanding systems
(Lada & Lada 2003), it is believed from theoretical approaches, that gas expulsion
affects the dynamical evolution of the remaining clusters significantly (Tutukov 1978;
Goodwin & Bastian 2006). In some SFRs, new generation stars are still forming in the
natal clouds swept away by the stellar wind and the UV radiation from high-mass stars
(e.g. Sicilia-Aguilar et al. 2004; Koenig et al. 2008; Lim et al. 2014b). The observed
structure is basically a projection onto the sky. Hence, it is difficult to determine the
physically meaningful extent of young open clusters. Alternatively, radial surface density
profile (RSDP) gives an area encompassing almost all the members of a cluster. This is a
